A common way of offset-calibrating an IR camera is to calibrate against an insertable spade. An example of a realization comprising a spade is described in US 2002/0162963 A1.
One problem with IR cameras provided with a spade and, in particular, with uncooled IR cameras is spatial noise (“fix pattern noise”, FPN) should the camera, and in particular the spade, not have the same temperature as the scene which the camera is viewing. Moreover, the spade with associated transport mechanisms constitutes a significant part of the production cost of the camera.
In order to get around the problem of a warm spade, it has previously been known to use the scene temperature in the calibration without being affected by objects in the scene. For this, it is necessary to be able to make a so-called NUC (Non Uniformity Correction). This requires the camera to be able to be directed towards a plane surface which has the same temperature as the scene. A surface which can be used in certain cases has been the sky.
It is also known to offset-calibrate an IR camera by moving one or more lenses in the camera in order to defocus the scene as much as possible and make the calibration against this almost uniform scene. The use of defocusing in connection with offset calibration is described, inter alia, in FR 2 928 462 A1. According to the said document, lenses are moved in the optical path for calibration by defocusing.
An offset calibration against the defocused scene removes, inter alia, the high-frequency noise and corrects for the characteristics of integral detectors. On the other hand, there remain problems with the stray light, since stray light in the defocused state looks different from in the focused state. This results in the addition of a new stray light, originating from the defocused state, to the stray light in the focused state.